With the recent years' development of electronics, there is a rapidly increasing demand for semiconductors for forming memories, integrated circuits, etc.
Such semiconductors are manufactured, for example, by doping a semiconductor substrate of a very high purity with impurities to impart electrical properties thereto, or by stacking together semiconductor substrates with minute circuit patterns formed thereon.
The operation of manufacturing such semiconductors must be conducted in a high vacuum chamber in order to avoid the influences of dust, etc. in the air. This chamber is generally evacuated by a vacuum pump. In particular, a turbo-molecular pump, which is a kind of vacuum pump, is widely used since it entails little residual gas and is easy of maintenance.
A semiconductor manufacturing process includes a number of steps in which various process gases are caused to act on a semiconductor substrate, and the turbo-molecular pump is used not only to evacuate the chamber but also to discharge these process gases from the chamber.
Further, in an apparatus like an electron microscope, a turbo-molecular pump is used to create a high vacuum state in the chamber of the apparatus in order to prevent refraction, etc. of an electron beam due to the presence of dust or the like.
Such a turbo-molecular pump is composed of a turbo-molecular pump main body for sucking and discharging gas from the chamber of a semiconductor manufacturing apparatus, an electron microscope, or the like, and a control device for controlling the turbo-molecular pump main body.
FIG. 8 is a longitudinal sectional view of a turbo-molecular pump main body.
In FIG. 8, a turbo-molecular pump main body 100 includes an outer cylinder 127, on top of which there is formed an intake hole 101. Provided inside the outer cylinder 127 is a rotor 103 having in its periphery a plurality of rotary blades 102a, 102b, 102c, . . . serving as turbine blades for sucking and discharging gas and formed radially in a number of stages.
At the center of the rotor 103, there is mounted a rotor shaft 113, which is supported in a levitating state and controlled in position, for example, by a so-called 5-axis control magnetic bearing.
Upper radial electromagnets 104 consist of four electromagnets arranged in pairs in X- and Y-axis directions, perpendicular to each other, and opposed to each other with the rotor shaft 113 therebetween. It is to be assumed that the X- and Y-axes are in a plane perpendicular to the axis of the rotor shaft 113 when the rotor shaft 113 is at a control target position of the magnetic bearing. Further, there is provided an upper radial sensor 107 consisting of four coils which are opposed to each other with the rotor 103 therebetween and arranged in close proximity to and in correspondence with the four upper radial electromagnets 104. The upper radial sensor 107 detects radial position of the rotor 103, transmitting a detection signal to a control device.
The rotor shaft 113 is formed of a high-magnetic-permeability material (e.g., iron) and is adapted to be attracted by the magnetic force of the upper radial electromagnets 104.
Further, lower radial electromagnets 105 and a lower radial sensor 108 are arranged in the same way as the upper radial electromagnets 104 and the upper radial sensor 107. The lower radial sensor 108 detects the radial position of the lower portion of the rotor shaft 113 and transmits a detection signal to the control device. The radial positions of the upper and lower portions of the rotor shaft 113 are adjusted by the magnetic bearing feedback control means in the control device.
Further, axial electromagnets 106A and 106B are arranged respectively on the upper and lower sides of a metal disc 111 provided in the lower portion of the rotor shaft 113. The metal disc 111 is formed of a high-magnetic-permeability material, such as iron. To detect axial position of the rotor 103, there is provided an axial sensor 109, which transmits an axial position signal to the control device.
The axial electromagnet 106A magnetically attracts the metal disc 111 upwardly, and the axial electromagnet 106B magnetically attracts the metal disc 111 downwardly.
In this way, in the control device, the magnetic force the axial electromagnets 106A and 106B exert on the metal disc 111 is appropriately controlled by the magnetic bearing feedback control means, magnetically levitating the rotor shaft 113 in the axial direction and retaining it in the space in a non-contact state.
A motor 121 is equipped with a plurality of magnetic poles consisting of permanent magnets arranged circumferentially on the rotor side so as to surround the rotor shaft 113. A torque for rotating the rotor shaft 113 is imparted to these permanent magnets from the electromagnets on the stator side of the motor 121, thereby rotating the rotor 103.
Further, an RPM sensor and a motor temperature sensor (not shown) are mounted to the motor 121, and the rotation of the rotor shaft 113 is controlled in the control device in response to detection signals from the RPM sensor and the motor temperature sensor.
A plurality of stationary blades 123a, 123b, 123c, . . . are arranged so as to be spaced apart from the rotary blades 102a, 102b, 102c, . . . by small gaps. In order to downwardly transfer the molecules of exhaust gas through collision, the rotary blades 102a, 102b, 102c, . . . are inclined by a predetermined angle with respect to a plane perpendicular to the axis of the rotor shaft 113.
Similarly, the stationary blades 123 are also inclined by a predetermined angle with respect to a plane perpendicular to the axis of the rotor shaft 113, and extend toward the inner side of the outer cylinder 127 to be arranged alternately with the rotary blades 102.
The stationary blades 123 are supported at one end by being inserted into gaps between a plurality of stationary blade spacers 125a, 125b, 125c, . . . stacked together in stages.
The stationary blade spacers 125 are ring-shaped members, which are formed of a metal, such as aluminum, iron, stainless steel, or copper, or an alloy containing such metal as a component.
In the outer periphery of the stationary blade spacers 125, the outer cylinder 127 is secured in position with a small gap therebetween. At the bottom of the outer cylinder 127, there is arranged a base portion 129, and a threaded spacer 131 is arranged between the lowermost one of the stationary blade spacers 125 and the base portion 129.
In the portion of the base portion 129 below the threaded spacer 131, there is formed a discharge outlet 133. Connected to the discharge outlet 133 is a dry-sealed vacuum pump passage (not shown), and the discharge outlet 133 is connected to a dry-sealed vacuum pump (not shown) through this dry-sealed vacuum pump passage.
The threaded spacer 131 is a cylindrical member formed of a metal, such as aluminum, copper, stainless steel, or iron, or an alloy containing such metal as a component, and has a plurality of spiral thread grooves 131a in its inner peripheral surface.
The spiral direction of the thread grooves 131a is determined such that when the molecules of the exhaust gas move in the rotating direction of the rotor 103, these molecules are transferred toward the discharge outlet 133.
Connected to the lowermost one of the rotary blades 102a, 102b, 102c, . . . of the rotor 103 is a cylindrical portion 102d, which extends vertically downwards. The outer peripheral surface of this cylindrical portion 102d sticks out toward the inner peripheral surface of the threaded spacer 131, and is in close proximity to the inner peripheral surface of the threaded spacer 131 with a predetermined gap therebetween.
The base portion 129 is a disc-like member constituting the base of the turbo-molecular pump main body 100, and is generally formed of a metal, such as iron, aluminum, or stainless steel. The base portion 129 physically retains the turbo-molecular pump main body 100, and also functions as a heat conduction passage. Thus, the base portion 129 is preferably formed of a metal that is rigid and of, high heat conductivity, such as iron, aluminum, or copper.
Further, connected to the base portion 129 is a connector 160, to which a cable is connected to electrically connect the turbo-molecular pump main body 100 and the control device.
In the above-described construction, when the rotary blades 102 are driven and rotated by the motor 121 together with the rotor shaft 113, an exhaust gas from a chamber (not shown) is sucked in through the intake hole 101, due to the action of the rotary blades 102 and the stationary blades 123.
The exhaust gas sucked in through the intake hole 101 passes between the rotary blades 102 and the stationary blades 123, and is transferred to the base portion 129. The exhaust gas transferred to the base portion 129 is sent to the discharge outlet 133 while being guided by the thread grooves 131a of the threaded spacer 131.
FIG. 9 is a block diagram of a motor driver circuit for driving the motor 121. A motor driver circuit 200 includes a three-phase bridge circuit 210 formed of six FET elements V18, V19, V20, V21, V22, and V23 in order to pass three phase current through a U-phase winding, a V-phase winding, and a W-phase winding of the stator of the motor 121 in accordance with the flow chart of FIG. 10. For example, in section 1, V18 and V21 are turned ON to pass current in the direction from the U phase to the V phase. Further, in section 2, V18 and V23 are turned ON to pass current in the direction from the U phase to the W phase. Also in the sections following section 3, the FET elements are sequentially switched in accordance with the flow chart to generate a rotating magnetic field.
One end of the three-phase bridge circuit 210 is connected to one end of a capacitor 203 through a short-circuit protection element 201. The other end of the three-phase bridge circuit 210 is connected to the other end of the capacitor 203 through a motor current detecting circuit 205. One end of the capacitor 203 is further connected to one end of a regenerative resistance 207 and a positive pole 209 of a power source. The other end of the capacitor 203 is connected to the other end of the regenerative resistance 207 through a regeneration resistance drive FET 211 and is connected to a negative pole 213 of the power source. Both ends of the regenerative resistance 207 are connected to a diode 215 in parallel with this regenerative resistance 207.
When accelerating the pump, a driver control CPU (not shown) performs PWM control on each FET of the three-phase bridge circuit 210 so that the value of current which is detected by the motor current detecting circuit 205 and is supplied to the motor 121 becomes a predetermined constant current value, while letting the current reversely flow through each phase in accordance with the timing of FIG. 10 by using a rotational speed sensor (not shown) so that the motor rotates at a predetermined rotation number. Further, when the motor is braked, the driver control CPU performs PWM control on each FET of the three-phase bridge circuit 210 so that the current has a predetermined current value, and makes the regenerative resistance 207 consume electric power regenerated from the motor 121 as heat energy with the current flowing through each phase in the direction reverse to that when accelerating the motor, thereby the regeneration resistance drive FET 211 being turned ON/OFF.
In order to consume more energy, the regenerative resistance 207 is a high-capacity (large-scaled) type, or is cooled by an air-cooling FAN mounted on a heat sink.
In order to let the regenerative resistance consume energy within a safe range, in a disclosed conventional technique, regeneration energy is calculated and whether or not to perform regenerative braking can be automatically selected with respect to each of a plurality of motors to prevent the regeneration energy from exceeding a predetermined value (Patent document 1). [Patent Document 1] Japanese Patent Laid-Open. Pub. No. 2006-194094